He attacked Mr Obama in Saturday's letter for not apologising to the Japanese people during a May trip to Hiroshima, describing the US president's speech there as "lacking stature."

He also wrote: "I want to express my deepest gratitude for the shows of respect, greetings and praise that I've received in recent days, which give me strength to reciprocate with ideas that I will send to party militants and relevant organisations."

"Modern medical techniques have allowed me to scrutinise the universe," wrote Castro, who stepped down as Cuba's president 10 years ago after suffering a severe gastrointestinal illness.

Castro accompanied his thanks with reminiscences about his childhood and youth in eastern Cuba, describing the geology and plant life of the region where he grew up.

The Cuban government has taken a relatively low-key approach to Castro's birthday, in comparison with the large-scale gatherings that had been planned for his 80th.

Government ministries have held small musical performances and photo exhibitions that pay tribute to the former head of state.

State media showed images of Venezuelan president Nicolas Maduro arriving in Havana and a tribute was planned at a Havana theatre for Sunday evening. The government did not say which Cuban officials would attend.

Castro last appeared in public in April, closing the twice-a-decade congress of the Cuban Communist Party with a call for Cuba to stick to its socialist ideals amid ongoing normalisation with the US.

A poster of Cuban Revolution leader Fidel Castro is seen on a wall in Havana, Cuba. Pictuer: AP Photo/Desmond Boylan

The need for closer economic ties with the US has grown more urgent as Venezuela, Castro's greatest ally, tumbles into economic free-fall, cutting the flow of subsidised oil that Cuba has depended on from the South American country for more than a decade.

Meanwhile, tens of thousands of Cubans are emigrating to the United States, hollowing out the ranks of highly educated professionals.

The brightest spot in Cuba's flagging economy has been a post-detente surge in tourism that is expected to boom when commercial flights to and from the United States, Cuba's former long-time enemy, resume on August 31.